Vehicle bi-directional power inverter system and method

ABSTRACT

An exemplary embodiment of the present invention provides a bi-directional inverter of a vehicle. The bi-directional inverter may include an alternating current (AC) to direct current (DC) inverter configured to receive AC power from a power grid and generate DC power on a DC bus operatively coupled to a vehicle battery. The bi-directional inverter may also include a DC to AC inverter configured to receive DC power from the DC bus and generate AC power delivered to the power grid. The bi-directional inverter may also include an energy management system operatively coupled to the AC to DC inverter and the DC to AC inverter and configured to selectively operate the bi-directional inverter in a charging mode or a generation mode. Additionally, the bi-directional inverter may include a power line communications (PLC) coupler configured to transfer electronic data between the energy management system and a power plant network through the power grid.

FIELD OF THE INVENTION

The present invention generally relates to a power system for a vehicle.More specifically, the present invention relates to a power system for avehicle that includes circuitry for selectively receiving power from anelectrical power grid or generating electrical power to be delivered tothe power grid. The power system can also be communicatively coupled toa power plant network through the electrical grid.

BACKGROUND OF THE INVENTION

This section is intended to introduce the reader to various aspects ofart which may be related to various aspects of the present inventionwhich are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

Electric vehicles have increased in popularity in recent years. Electricvehicles and plug-in hybrid electric vehicles may be useful for reducingdependency on fossil fuels and increasing fuel efficiency. Electric andplug-in electric vehicles generally receive electrical power through apower grid provided by an electric utility. Thus, a typical electricvehicle may include an AC to DC inverter for receiving AC power from thegrid to charge the vehicle battery.

SUMMARY OF THE INVENTION

An exemplary embodiment of the present invention provides abi-directional inverter of a vehicle. The bi-directional inverter mayinclude an alternating current (AC) to direct current (DC) inverterconfigured to receive AC power from a power grid and generate DC poweron a DC bus operatively coupled to a vehicle battery. The bi-directionalinverter may also include a DC to AC inverter configured to receive DCpower from the DC bus and generate AC power delivered to the power grid.The bi-directional inverter may also include an energy management systemoperatively coupled to the AC to DC inverter and the DC to AC inverterand configured to selectively operate the bi-directional inverter in acharging mode or a generation mode. Additionally, the bi-directionalinverter may include a power line communications (PLC) couplerconfigured to transfer electronic data between the energy managementsystem and a power plant network through the power grid.

In some embodiments, the bi-directional inverter may include a secondPLC coupler communicatively coupled to a vehicle network, the vehiclenetwork configured to receive electronic communications from the powerplant network through the power grid. In such embodiments, a userinterface operatively coupled to the vehicle network and configured toenable a user to interface with the energy management system.Interfacing with the energy management system may include generating acharge/generation schedule based, at least in part, on an electricityrate provided by the power plant network through the power grid andstoring the charge/generation schedule to the energy management system.Furthermore, in some embodiments, the electronic communications betweenthe power plant network, the vehicle network, and the energy managementsystem are conducted through a TCP/IP-based communications protocol.

In some exemplary embodiments, the energy management system isconfigured to automatically generate a charge/generation schedule based,at least in part, on electricity rates received from the power plantnetwork through the power grid. In some exemplary embodiments, an inputof the AC to DC inverter is operatively coupled to a vehicle ACgenerator electrically that is coupled in series between the power gridand the AC to DC inverter and configured to power the DC bus through theAC to DC inverter. In some exemplary embodiments, the DC to AC invertercomprises a DC switching circuit configured to generate a sinusoidaloutput waveform.

Another exemplary embodiment of the present invention provides a vehiclethat includes a battery configured to provide power to a vehiclepropulsion system. The vehicle also includes an AC to DC inverterconfigured to receive AC power from a power grid and generate DC poweron a DC bus. The vehicle also includes a DC to AC inverter configured toreceive DC power from the DC bus and generate AC power delivered to thepower grid. The vehicle also includes an energy management systemoperatively coupled to the AC to DC inverter and the DC to AC inverterand configured to selectively operate the bi-directional inverter in acharging mode or a generation mode. Furthermore, the energy managementsystem is configured to communicate with a power plant network throughthe power grid.

In some exemplary embodiments, the vehicle may also include a vehiclenetwork communicatively coupled to the power grid for communicating withthe power plant network and the energy management system. In suchembodiments, a first PLC coupler may be configured to transferelectronic data between the energy management system and the power gridand a second PLC coupler may be configured to transfer electronic databetween the vehicle network and the power grid. Furthermore, theelectronic communications between the power plant network, the vehiclenetwork, and the energy management system may be conducted through aTCP/IP-based communications protocol.

In some exemplary embodiments, the vehicle may include a user interfacecommunicatively coupled to the vehicle network, wherein the userinterface may be used to initiate the charging mode and the generationmode through the energy management system. In some exemplaryembodiments, the user interface may be configured to display informationreceived from the power plant network, the information including anelectricity rate schedule. In some exemplary embodiments, a globalpositioning system (GPS) navigation system is communicatively coupled tothe vehicle network and configured to send travel data to the energymanagement system, wherein the energy management system is configured toautomatically determine a charge/generation schedule, based, at least inpart, on the travel data.

Another exemplary embodiment of the present invention provides a methodof managing power usage in a vehicle. The method may include receivingelectronic data from a power plant network through a power grid andswitching a power system of the vehicle to a generation mode or chargingmode based, at least in part, on the electronic communications receivedfrom the power plant network through the power grid. The generation modecauses the vehicle to draw DC electrical power from a vehicle batteryand generate an AC output power delivered to the power grid. Thecharging mode causes the vehicle to draw AC electrical power from thepower grid and generate DC electrical power for charging the vehiclebattery. In such exemplary embodiments, the receiving the electronicdata may include receiving an electricity rate from the power plantnetwork or receiving an instruction from the power plant networkinstructing the power system of the vehicle to initiate or terminate acharge mode or generation mode depending, at least in part, on acombined electrical demand on the power grid.

In some exemplary embodiments, the method may include determining acharge/generation schedule based, at least in part, on an electricityrate schedule provided by the power plant network through the powergrid. In some exemplary embodiments, the method may include switching apower system of the vehicle to a generation mode or charging mode based,at least in part, on input provided by a user through a user interfacecommunicatively coupled to the power grid.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of the presentinvention, and the manner of attaining them, will become apparent and bebetter understood by reference to the following description of oneembodiment of the invention in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a block diagram of a vehicle power system 100 with abi-directional inverter 102, in accordance with an exemplary embodimentof the present invention; and

FIG. 2 is a process flow chart illustrating a method 200 for operating apower system of a vehicle, in accordance with an exemplary embodiment.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate a preferred embodiment of the invention, in one form, andsuch exemplifications are not to be construed as limiting in any mannerthe scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions may be made toachieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

Exemplary embodiments of the present invention relate to a power systemfor an electric vehicle, for example, a plug-in hybrid electric vehicle,and the like. The power system may include a bi-direction inverter thatis capable of operating in a charging mode and a generation mode. Duringthe charging mode the bi-direction inverter provides AC to DC conversionfor charging a vehicle battery(s) from an electrical grid. Duringgeneration mode the bi-direction inverter provides DC to AC conversionfor generating power that is delivered back to the electrical grid. Thepower system may also include control circuitry for selectivelyswitching the power system between the charging mode and a generationmode. Additionally, the power system may communicate with acommunications network of an electricity provider to receive a varietyof information, such as electricity rates. Information received from thecommunications network may be used by the control circuitry to determinewhether to operate the power system in the charging mode or generationmode. Furthermore, the power system may include PLC circuitry thatenables the power system to communicate with the communications networkthrough an electrical power grid such as Smart Grid. As used herein, theterm “Smart Grid” is used to refer to an electrical grid that enables anelectric utility to manage power usage of devices coupled to theelectrical grid by communicating with the devices through the electricalgrid.

FIG. 1 is a block diagram of a vehicle power system 100 with abi-directional inverter 102, in accordance with an exemplary embodiment.The bi-directional inverter 102 may be coupled to a power grid 104.Accordingly, the bi-directional inverter 102 may include an electricalconnector that enables the user of the vehicle to couple the powersystem 100 to the power grid 104 when the vehicle is stationary. Thepower grid 104 may be any suitable electrical distribution networkprovided, for example, by an electric power utility or other electricalgeneration and distribution facility. In some embodiments, theelectrical grid may be a Smart Grid that provides electrical power aswell as electronic communications.

The bi-directional inverter 102 may be coupled to a vehicle battery 106that is used to store energy used by a vehicle propulsion system such asan electric motor. The vehicle battery 106 may be any suitable vehiclebattery, for example, a 420 Volt lithium fluoride battery. Furthermore,the vehicle battery 106 may include a plurality of batteries. Thebi-directional inverter 102 may also be coupled to a vehicle ACgenerator 108, such as an alternator, which may generate single-phase ACpower used to charge the battery 106, for example, when the vehicle ismobile. Additionally, when the vehicle in coupled to the power grid 104and operating in generation mode, the vehicle AC generator 108 may beused to provide the electrical power that is delivered to the power grid104.

The bi-directional inverter 102 may include an AC to DC inverter 110,for converting AC power received from the power grid 104 or the vehicleAC generator 108 into DC electrical power that is delivered to a DC bus112. The AC to DC inverter 110 may include any suitable circuitry forconverting AC power into DC power, for example, a step-up transformer, arectifier, and the like. In some embodiments, the AC to DC inverter 110may include a switched-mode power supply, a silicon-controlled rectifier(SCR) for a crowbar protection circuit, a bridge rectifier, and thelike. In a switched-mode configuration, the AC to DC inverter 110 mayinclude solid-state switches such as metal-oxide semiconductorfield-effect transistors (MOSFETs) for a DC boost circuit. The AC to DCinverter 110 may also include circuitry for reducing noise on the DC bus112, for example, capacitors, inductors, and the like. The input of theAC to DC inverter 110 may be coupled to the power grid 104 through asingle-phase 110 Volt electrical connection, as shown in FIG. 1, whichis generally available in most vehicle garages. However, various otherelectrical configurations may also be used to couple the AC to DCinverter 110 to the power grid 104. Additionally, an input of the AC toDC inverter 110 may also be coupled to the output of the vehicle ACgenerator 108 in a single-phase configuration, as shown in FIG. 1. Theoutput of the AC to DC inverter 110 may be coupled to the DC bus 112,which is also coupled to the battery 106. During charge mode, the AC toDC inverter 110 receives AC power from the power grid 104 and providesDC power to the DC bus 112 for charging the vehicle battery 106.

The bi-directional inverter 102 may also include a DC to AC inverter 114for generating AC power that may be delivered to the power grid 104. TheDC to AC inverter 114 may include any suitable AC inverter forconverting the DC power provided by the DC bus 112 into AC power thatmay be delivered to the electrical grid 104. For example, the DC to ACinverter 114 may include an SCR inverter, a insulated gate bipolartransistor (IGBT) inverter, a silicon-carbide Field Effect Transistor(SiC FET) inverter, a gallium nitride Metal Semiconductor Field EffectTransistors (GaN MESFET) inverter, as well as other rectifiers thatutilize high-power semiconductor switching devices. In some embodiments,the DC to AC inverter 114 may be pulse width modulated. In suchembodiments, the DC to AC inverter 114 may generate a sinusoidal outputwaveform that may reduce electromagnetic interference in thebi-directional inverter 102 as compared to a square wave output. In thisway, a signal-to-noise ratio of the electronic data transmitted over thepower grid 104 or within the bi-directional inverter 102 may be reduced.The input of the DC to AC inverter 114 may be coupled to the output ofthe AC to DC inverter 110 through the DC bus 112. The output of the DCto AC inverter 114 may be optionally coupled to the power grid 104.During generation mode, the DC to AC inverter 114 may receive DC powerfrom the DC bus 112 and deliver AC power to the power grid 104. Forexample, the DC to AC generator may provide 2 to 10 Kilowatt, 110 VoltAC power to the power grid 104. Furthermore, during generation mode, theDC bus 112 may be powered by the battery 106 or the vehicle AC generator108.

A number of PLC coupler 116 in the bi-directional inverter 102 may serveas a data interface between the power grid 104 and various electronicdevices included in the bi-directional inverter 102. The PLC coupler 116may provide high-speed data transmission over the power grid 104, forexample, 200 to 400 megabit per second. In some exemplary embodiments,the PLC coupler 116 may comprise an HD-PLC coupler available fromPanasonic Corporation. Communication between the power grid 104 and thedevices in the bi-directional inverter 102 may be based on any of alarge number of network technologies. The specific network technologychosen for a given application may vary based on design considerationsfor the specific application. By way of example, a TCP/IP communicationsprotocol may be used. In some embodiments, a fixed Internet protocol(IP) address may be assigned to the vehicle. In this way, a user'svehicle may be easily identified through the power grid 104.

An energy management system 118 may be included in the bi-directionalinverter 102 for controlling its operating mode. The energy managementsystem 118 may include a processor, a tangible machine-readable memory,and other circuitry used to selectively switch the bi-directionalinverter 102 to the charge mode or generation mode. Accordingly, theenergy management system 118 may send control signals to the AC to DCinverter 110 and the DC to AC inverter 114. For example, the energymanagement system 118 may send switch control signals to the DC to ACinverter 114 to generate the AC power delivered to the power grid 104.Additionally, the energy management system 118 may send switch controlsignals to the AC to DC inverter 110 to generating the DC voltage outputto the DC bus 112.

The energy management system 118 may be communicatively coupled to thepower grid 104 through a PLC coupler 116. In some embodiments, a router120 may pass data between the energy management system 118 and the PLCcoupler 116. The router 120 may be an Ethernet router for transmittingTCP/IP packet information to and from the power grid 104 through the PLCcoupler 116. The energy management system 118 may be communicativelycoupled through the power grid 104 to a power plant network 122, whichserves as a communications center of the power plant. The power plantnetwork 122 may be communicatively coupled to the power grid 104 throughanother PLC coupler 116 and router 120 combination, as shown in FIG. 1.

Through the power grid 104, the energy management system 118 may receivedata from the power plant network 122. For example, the energymanagement system 118 may receive data that relates to electrical rates,electricity availability, and the like. The energy management system 118may use data received from the power plant network 122 to determine theoperating mode of the bi-directional inverter 102. For example, theenergy management system 118 may initiate the charge mode duringoff-peak electricity usage periods, during which the overall demand onthe power grid 104 may be lower and the electricity rates may bereduced. The energy management system 118 may initiate generation modeduring electrical shortages or during peak electricity usage periods,during which the demand on the power grid 104 may be higher and theelectricity rates increased; thus, increasing the amount of moneycredited back to the customer.

In some exemplary embodiments, the energy management system 118 may alsoreceive operational commands from the power plant network 122. Forexample, during electricity shortages the power plant network 122 maysend commands to the energy management system 118 instructing the energymanagement system 118 to terminate the charge mode, activate thegeneration mode, or vice versa.

In some exemplary embodiments, the energy management system 118 maygenerate a log of various energy usage characteristics of thebi-directional inverter 102. For example, the log may includeinformation such as battery charge history, energy usage history of thevehicle, and the like. The energy management system 118 log may alsoinclude details of prior charge/generation periods, such as the amountof power received from or delivered to the power grid 104, theelectricity rates incurred from or charged to the electrical utility,and the like. Information in the log may be viewed by the vehicle user,as described below.

In some exemplary embodiments, the bi-directional inverter 102 may becoupled to a vehicle network 124. In some embodiments, the vehiclenetwork 124 may include a PLC bus that uses a TCP/IP basedcommunications protocol and provides both data communications and DCpower to the devices coupled to the vehicle network 124. The vehiclenetwork 124 may be coupled to the to the power grid 104 through a PLCcoupler 116 and network interface controller (NIC) 126, as shown inFIG. 1. In some embodiments, the NIC 126 may be an Ethernet-over-power(EOP) adapter that provides data communications over the PLC bus.

The vehicle network 124 may provide connectivity between a variety ofdevices in the vehicle. For example, the vehicle network 124 may provideelectronic communications between various media devices, such as a DVDplayer, a vehicle audio system, rear view camera, vehicle instrumentcluster, global positioning system (GPS) navigation system, a wirelessnetwork, and the like. Additionally, the vehicle network 124 may alsoenable devices coupled to the vehicle network 124 to communicate withthe energy management system 118 and the power plant network 122.

In some exemplary embodiments, the vehicle network 124 may be coupled toa user interface 128 that enables the vehicle user to manage the energyusage of the vehicle. In some exemplary embodiments, the user interface128 may be provided in a vehicle infotainment interface. As used herein,the term “infotainment interface” refers to an in-vehicle informationand entertainment system that combines a fixed user interface withinformation and entertainment sources located in the vehicle, such as avehicle audio system, DVD player, GPS navigation system, and the like.In some embodiments, the user interface 128 may receive data from thepower plant network 122 through the power grid 104. For example, theuser interface 128 may receive data about current or future expectedelectricity rates, rate schedules, and the like. The user interface 128may also receive information regarding the user's account with theelectrical utility. For example, the user interface 128 may receiveinformation such as an account statement, an amount due for electricityused or owed for electricity provided, and the like. The connection tothe power plant network 122 may also enable the user to manage theaccount, for example, paying an outstanding balance, changing a rateplan, and the like.

In some exemplary embodiments, the user interface 128 may also be usedto communicate with the energy management system 118 through the PLCcoupler 116 coupled to the power grid 104. In some embodiments, theenergy management system 118 may provide energy management data to theuser interface 128. For example, the energy management system 118 maysend data to the user interface 128 regarding the current operating modeof the bi-directional inverter 102, current battery charge, and thelike. Additionally, the energy management data may include informationstored to the log generated by the energy management system 118 asdiscussed above. In this way, the user may be able to view data relatedto the energy usage of the vehicle, for example, time and duration ofprevious charge/generation periods, the electricity rate applied duringprevious charge/generation periods, and the like.

The user interface 128 may also be used to manage a charge/generationschedule that may, at least in part, control when the energy managementsystem 118 initiates or terminates the charge and generation modes. Insuch embodiments, the user may view a rate schedule provided by thepower plant network 122, the rate schedule indicating the electricityrates charges for different days or different times of day. Based onthis information and future expected energy needs, the user may manuallycreate or alter the charge/generation schedule through the userinterface 128. The charge/generation schedule may be stored to theenergy management system 118. In some embodiments, the energy managementsystem 118 may be generated automatically by the energy managementsystem 118, based on the information provided by the power plant network122. In such embodiments, the energy management system 118 may beconfigured to provide the optimize power usage, based on the electricityrates indicated by the power plant network 122. For example, the energymanagement system 118 may be configured to initiate the charging modewhen rates are low and initiate the generation mode when rates are high.

In some exemplary embodiments, the energy management system 118 may alsoreceive travel data from the instrument cluster or the GPS navigationsystem coupled to the vehicle network 124. For example, the travel datamay include a recorded driving history, for example, informationregarding prior trips such as distance, driving time, electricity usage,average speed, vehicle usage periods, and the like. The travel data mayalso include future trips, which may be received from a trip-planningfeature of the GPS navigation system. The driving history and futuretrips may be used by the energy management system 118 to estimate futureenergy needs and automatically determine a charge/generation schedulethat optimizes energy usage. For example, if the driving history orfuture trips suggest that the vehicle is likely to use a large amount ofelectricity, the energy management system 118 may compute acharge/generation schedule that provides a full battery charge when thevehicle will likely to be used. If the driving history or future tripssuggest minimal vehicle usage such as short trips to and from work, theenergy management system 118 may compute a charge/generation schedulethat sells excess stored battery charge back to the power grid 104, forexample, during peak electricity usage periods. The charge/generationschedule automatically generated by the energy management system 118 mayalso be manually altered by the user through the user interface 128.

FIG. 2 is a process flow chart illustrating a method 200 for operating apower system of a vehicle, in accordance with an exemplary embodiment.The method may begin at block 202, wherein the bi-directional inverter102 receives electronic data from the power plant network 122 throughthe power grid 104. For example, as discussed above, the bi-directionalinverter 102 may receive information about electricity rates, such asthe current electricity rate, an electricity rate schedule, and thelike. In some embodiments, the bi-directional inverter 102 may receiveinstructions from the power plant network 122 that instruct thebi-directional inverter 102 to initiate or terminate the charge mode orgeneration mode. For example, the power plant may instruct thebi-directional inverter 102 to terminate charge mode or initiategeneration mode if the combined demand on the power grid 104 exceeds theelectrical generation capability of the power plant.

The process flow may then advance to block 204, wherein thebi-directional inverter 102 may be switched to a generation mode orcharging mode based, at least in part, on the electronic communicationsreceived from the power plant network 122 through the power grid 104.For example, the bi-directional inverter 102 may switch to a generationmode or charging mode in response to a command from the power plantnetwork 122 to switch to the corresponding mode. In embodiments whereinthe data received from the power plant network 122 is a currentelectricity rate, the bi-directional inverter 102 may switch to ageneration mode or charging mode based on the electricity rate. Forexample, if the current electricity rate rises above a specifiedthreshold, the bi-directional inverter 102 may switch to a generationmode. Conversely, if the current electricity rate falls below aspecified threshold, the bi-directional inverter 102 may switch to acharging mode. The specified thresholds may be specified by the user andprogrammed into energy management system 118. Furthermore, inembodiments wherein the data received from the power plant network 122includes future electricity rates, the future electricity rates may beused to generate a charge/generation schedule, as discussed above inrelation to FIG. 1. The energy management system 118 may then switch thebi-directional inverter to the charge mode or the generation mode inaccordance with the charge/generation schedule.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

What is claimed is:
 1. A vehicle, comprising: a vehicle power systemincluding a bi-directional inverter, the bi-directional inverter having:an alternating current (AC) to direct current (DC) inverter configuredto receive AC power from a power grid and generate DC power on a DC busoperatively coupled to a vehicle battery; a DC to AC inverter configuredto receive DC power from the DC bus and generate AC power delivered tothe power grid; an energy management system operatively coupled to theAC to DC inverter and the DC to AC inverter and configured toselectively operate the bi-directional inverter in a charging mode or ageneration mode; and a power line communications (PLC) couplerconfigured to transfer electronic data between the energy managementsystem and a power plant network through the power grid; wherein theenergy management system is configured to automatically generate acharge/generation schedule based, at least in part, on electricity ratesreceived from the power plant network through the power grid.
 2. Thevehicle of claim 1, wherein the bi-directional inverter comprises asecond PLC coupler communicatively coupled to a vehicle network, thevehicle network configured to receive electronic communications from thepower plant network through the power grid.
 3. The vehicle of claim 2,wherein the bi-directional inverter comprises a user interfaceoperatively coupled to the vehicle network and configured to enable auser to interface with the energy management system.
 4. The vehicle ofclaim 3, wherein interfacing with the energy management system comprisesgenerating a charge/generation schedule based, at least in part, on anelectricity rate provided by the power plant network through the powergrid and storing the charge/generation schedule to the energy managementsystem.
 5. The vehicle of claim 2, wherein the electronic communicationsbetween the power plant network, the vehicle network, and the energymanagement system are conducted through a TCP/IP-based communicationsprotocol.
 6. The vehicle of claim 1, wherein an input of the AC to DCinverter is operatively coupled to a vehicle AC generator electricallythat is coupled in series between the power grid and the AC to DCinverter and configured to power the DC bus through the AC to DCinverter.
 7. The vehicle of claim 1, wherein the DC to AC invertercomprises a switched-mode power supply configured to generate asinusoidal output waveform.
 8. The bi-directional inverter of claim 1,wherein the electricity rates include a schedule of rates of monetarycosts of electricity.